Coagulation is a physiological pathway involved in maintaining normal blood hemostasis in mammals. Under conditions in which a vascular injury occurs, the coagulation pathway is stimulated to form a blood clot to prevent the loss of blood. Immediately after the vascular injury occurs, blood platelets begin to aggregate at the site of injury forming a physical plug to stop the leakage. In addition, the injured vessel undergoes vasoconstriction to reduce the blood flow to the area and fibrin begins to aggregate forming an insoluble network or clot, which covers the ruptured area. When an imbalance in the coagulation pathway shifts towards excessive coagulation, the result is the development of thrombotic tendencies, which are often manifested as heart attacks, strokes, deep vein thrombosis, and myocardial infarcts. Current therapies for treating disorders associated with imbalances in the coagulation pathway involve many risks and must be carefully controlled.
Heparin, a highly sulphated heparin-like glycosaminoglycan (HLGAG) produced by mast cells, is a widely used clinical anticoagulant, and is one of the first biopolymeric drugs and one of the few carbohydrate drugs. Heparin primarily elicits its effect through two mechanisms, both of which involve binding of antithrombin III (AT-III) to a specific pentasaccharide sequence, HNAc/S,6SGHNS,3S,6SI2SHNS,6S contained within the polymer. First, AT-III binding to the pentasaccharide induces a conformational change in the protein that mediates its inhibition of factor Xa. Second, thrombin (factor IIa) also binds to heparin at a site proximate to the pentasaccharide AT-III binding site. Formation of a ternary complex between AT-III, thrombin and heparin results in inactivation of thrombin. Unlike its anti-Xa activity that requires only the AT-III pentasaccharide-binding site, heparin's anti-IIa activity is size-dependant, requiring at least 18 saccharide units for the efficient formation of an AT-III, thrombin, and heparin ternary complex.
In addition to heparin's anticoagulant properties, its complexity and wide distribution in mammals have lead to the suggestion that it may also be involved in a wide range of additional biological activities. Heparin-like glycosaminoglycans (HLGAGs), present both at the cell surface and in the extracellular matrix, are a group of complex polysaccharides that are variable in length, consisting of a disaccharide repeat unit composed of glucosamine and an uronic acid (either iduronic or glucuronic acid). The high degree of complexity for HLGAGs arises not only from their polydispersity and the possibility of two different uronic acid components, but also from differential modification at four positions of the disaccharide unit. Three positions, viz., C2 of the uronic acid and the C3, C6 positions of the glucosamine can be O-sulfated. In addition, C2 of the glucosamine can be N-acetylated or N-sulfated. Together, these modifications could theoretically lead to 32 possible disaccharide units, making HLGAGs potentially more information dense than either DNA (4 bases) or proteins (20 amino acids). This enormity of possible structural variants allows HLGAGs to be involved in a large number of diverse biological processes, including angiogenesis (Sasisekharan, R., Moses, M. A., Nugent, M. A., Cooney, C. L. & Langer, R. (1994) Proc Natl Acad Sci USA 91, 1524-8.), embryogenesis (Binari, R. C., Staveley, B. E., Johnson, W. A., Godavarti, R., Sasisekharan, R. & Manoukian, A. S. (1997) Development 124, 2623-32; Tsuda, M., Kamimura, K., Nakato, H., Archer, M., Staatz, W., Fox, B., Humphrey, M., Olson, S., Futch, T., Kaluza, V., Siegfried, E., Stam, L. & Selleck, S. B. (1999) Nature 400, 276-80.; and Lin, X., Buff, E. M., Perrimon, N. & Michelson, A. M. (1999) Development 126, 3715-23.) and the formation of β-fibrils in Alzheimer's disease (McLaurin, J., Franklin, T., Zhang, X., Deng, J. & Fraser, P. E. (1999) Eur J Biochem 266, 1101-10. And Lindahl, B., Westling, C., Gimenez-Gallego, G., Lindahl, U. & Salmivirta, M. (1999) J Biol Chem 274, 30631-5).
Although heparin is highly efficacious in a variety of clinical situations and has the potential to be used in many others, the side effects associated with heparin therapy are many and varied. Side effects such as heparin-induced thrombocytopenia (HIT) are primarily associated with the long chain of un-fractionated heparin (UFH), which provides binding domains for various proteins. This has lead to the explosion in the generation and utilisation of low molecular weight heparin (LMWH) as an efficacious alternative to UFH. Although attention has been focused on LMWH as heparin substitutes due to their more predictable pharmacological action, reduced side effects, sustained antithrombotic activity, and better bioavailability, there is at present limited ability to standardize the LMWH manufacturing process. Because the LMWH are derived from heparins and hence are polydisperse and microheterogenous, with undefined structure, they possess inherent variability, which currently prevents an efficient process for their manufacture. It would be of value both medically and scientifically to have a consistent, quality controlled, time efficient, concentration independent, and highly reproducible method for producing heparin and other glycosaminoglycans.
In an attempt to characterize the molecular, structural, and activity variations of heparin, several techniques have been investigated for the analysis of heparin preparations. Gradient polyacrylamide gel electrophoresis (PAGE) and strong ion exchange HPLC (SAX) have been used for the qualitative and quantitative analysis of heparin preparations. Although the gradient PAGE method can be useful in determining molecular weight, it suffers from the lack of resolution, particularly the lack of resolution of different oligosaccharides having identical size. SAX-HPLC, which relies on detection by ultraviolet absorbance, is often insufficiently sensitive for detecting small amounts of structurally important heparin-derived oligosaccharides. The current technologies for purifying and analyzing heparins and other glycosaminoglycans are insufficient. There is a great clinical and scientific need for improved isolation and analysis methods.